Photonic bandgap structures are currently being investigated for electromagnetic (EM) wave applications. Such photonic bandgap structures have a two- or three-dimensional periodic array structure in which the propagation of EM waves is governed by band-structure types of dispersion relationships. These photonic bandgap structures provide electromagnetic analogs to electron-wave behavior in crystals, with electron-wave concepts such as reciprocal space, Brillouin zones, dispersion relations, Bloch wave functions, van Hove singularities and tunneling having electromagnetic counterparts in photonic bandgap structures. This will enable the development of many new and improved types of photonic bandgap devices, including devices in which optical modes, spontaneous emission, and zero-point fluctuations are greatly reduced or inhibited. Photonic bandgap structures can also be formed with local disturbances in the periodic array structure, thereby generating defect or cavity modes with frequencies within a forbidden bandgap, for use in forming high-Q resonators or filters.
Photonic bandgap structures can be formed for use in the microwave or millimeter-wave regions of the spectrum by conventional machining processes (e.g. drilling or milling processes) or by laser ablative machining. For applications in the micron to sub-micron wavelength ranges, there is a need for improved fabrication processes that overcome accuracy and reproducibility limitations of conventional machining and laser ablation. For forming such micron or sub-micron range photonic bandgap structures, microelectronic fabrication processes are to be preferred as suggested, for example, in U.S. Pat. No. 5,335,240 to Ho et al.
However, variations in topography have been problematic when trying to form photonic bandgap structures using microelectronic fabrication processes heretofore. Successive deposition and patterning steps can result in an increasingly severe topography which builds up as each succeeding layer of a photonic bandgap structure is formed using conventional microelectronic deposition and patterning processes. This can limit the number of layers in a photonic bandgap structure formed by microelectronic processes or otherwise degrade performance of the completed structure due to variations in vertical dimensioning of the elements formed within different layers of the structure. What is needed is a method for fabricating a photonic bandgap structure that provides for each, and preferably every, layer of elements in the structure to be formed to precise vertical tolerances and that substantially eliminates any topographic buildup during formation of the structure.
An advantage of the fabrication method of the present invention is that a chemical-mechanical polishing step can be used to planarize each layer of a photonic bandgap structure, thereby providing a precise vertical tolerancing for the layer and eliminating any buildup of topographic variations across the layer.
Another advantage of the fabrication method of the present invention is that photonic bandgap structures can be formed with elements having critical dimensions in the range of several tens of microns down to a fraction of a micron (i.e. sub-micron).
A further advantage of the fabrication method of the present invention is that a photonic bandgap structure having a large area of several square centimeters or more can be formed with substantially uniform characteristics.
Yet another advantage is that microelectronics batch processing can be used to form a large of photonic bandgap structures with substantially identical characteristics.
Still another advantage is that the fabrication method of the present invention can be adapted to form either two- or three-dimensional photonic bandgap structures.
These and other advantages of the method of the present invention will become evident to those skilled in the art.